A typical safety and pressure relief valve is designed on the basis of a convergent-divergent nozzle flow concept. At full lift, i.e. where the distance between the valve poppet and the valve seat is maximum, the flow-controlling cross sectional area is the minimum cross-sectional area of a convergent nozzle positioned upstream of the seat. Thus, choking occurs inside the throat of the convergent nozzle, and a supersonic flow field (Mach&gt;1.0), including shock waves, via radial pressure distributions, actuates the valve poppet or disk and maintains it open during the discharge of the fluid.
This typical safety and pressure relief valve produces a non steady flow which causes popping action of safety and the moving parts at set pressure and hysteresis of the lifting forces versus the vessel pressure. Furthermore, the opening forces at the same vessel pressure are greater during valve closing than during valve opening.
In an attempt to control the popping action of safety and pressure valves, some manufacturers have incorporated "huddling chambers" in various valves. This valve design leads to the following problems:
different categories and series of valves have parts which differ geometrically depending on the type of fluid (e.g. gas, steam or liquid), admissible overpressure and blowdown values;
the parts for each valve size within a series are usable only with a completely assembled valve;
valves are large relative to nozzle bore size due to fluid expansion which results in a supersonic flow field during lifting of the valve;
one to three adjustable rings within the valve need manual adjustments in order to satisfy valve performance requirements;
pressure ranges are limited for any given spring rate, for example the pressure range in which a required performance can be obtained with a particular adjustment;
valves must be set and adjusted on flow test rigs due to the intrinsic interdependence of the popping pressure and blowdown value, as adjustment of the blowdown changes the set (popping) pressure without any change in the spring load;
valve stability is reduced due to a flowforce vs. lift (at constant vessel pressure) having slope sign changes, low damping factors and non-steady transonic flowfield with shocks under the moving parts;
large valve bowl volumes are required in order to increase the fill-up time and avoid simultaneous valve bowl pressure increases upon sudden opening of the valve;
choking can occur in the valve bowl exit orifice, thereby increasing the pressure acting in the closing direction on top of the moving parts and thus reducing the discharge capacity and stability of the valve; and
inlet and outlet headers reduce the valve performance to an extent requiring strict installation limitations.